1. Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for measuring characteristics of a deformable object through changes in the surface of the object during a deformation interval. More particularly, embodiments of the invention relate to the measurement of physical and biomechanical characteristics of a live cornea.
2. Description of Related Art
The measurement of the surface characteristics of an object can reveal much information about the physical and mechanical properties of the object. If the surface of the object is deformable in response to an applied force, measurement of the changes in characteristics of the surface may provide further useful information. There exists numerous organic and inorganic objects having deformable surfaces whose measurement may be of interest in various fields. A particularly interesting, exemplary object is the cornea of a human eye. The widespread interest in understanding the physical, biomechanical, optical and all other characteristics of the eye is obviously motivated. Over the years, different theories have been presented about the structural and dynamic properties of the eye, particularly the cornea. Earlier theories modeling the cornea as a solid structure have more recently given way to understanding the cornea as a layered, biodynamically responsive structure that to this day is not completely understood.
Increased understanding of the structure of the cornea and its interaction with other components of the eye has been achieved by measuring various topographical characteristics of the cornea. These topographical characteristics include corneal curvature and surface elevation with respect to a reference surface, as well as others known in the art. Corneal topography measuring devices are alternatively referred to as topographers, keratographers or keratometers (a topographer is a generic term referring to an apparatus for measuring the topographical characteristics of an object surface, while keratographer and keratometer more specifically refer to measurements of the cornea). Different devices use different measuring principles to determine various topographical characteristics of the cornea. For example, some devices use Placido-based reflective image analysis. Placido-based devices can measure curvature parameters of the cornea but typically lack the capability to directly measure surface elevation. The Orbscan® anterior segment analyzer (Bausch & Lomb Incorporated) is a topography characteristic measuring device that utilizes a scanning optical slit. Device software provides for direct measurement of surface elevation and corneal thickness as well as surface curvature. Another commercial device developed by Par Technology Corporation is known as the PAR CTS™ Corneal Topography System (PAR). The PAR imaging system utilizes a raster photography method. The PAR CTS imaging system projects a known grid geometry onto the anterior corneal surface that is viewed by a camera from an offset axis. Other topography characteristic measuring techniques include confocal microscopy, optical coherence tomography, ultrasound, optical interferometry and others, all of which are well known in the art.
While the measurement of various topographical characteristics of the cornea provide a wealth of information about vision and the effects of corneal shape on visual performance, corneal topography by itself cannot reveal the physical and biomechanical properties of the cornea necessary for a thorough understanding of its structure and function. In order to better understand the biomechanical and biodynamic properties of the cornea, it is necessary to know something about the elastic and viscoelastic properties of the cornea. One technique used to explore these properties is to deform the cornea with a known force and measure the response of the cornea to the force. An illustrative apparatus of this type is known in the art as a tonometer. Tonometers for measuring intraocular pressure (IOP) where originally developed as contact-type instruments, meaning that a portion of the instrument is brought into contact with the cornea during the measurement procedure. A well known instrument of this type is the Goldmann applanation tonometer (GAT) originally developed in the 1950s. The GAT measures the force required to flatten (“applanate”) a known area of the cornea, and is used today as a standard against which other types of tonometers are compared to assess measurement accuracy.
Patient discomfort caused by contact tonometers such as the GAT led to the development of “non-contact” tonometers, which operate by directing an air pulse generated by a pump mechanism through a discharge tube aimed at the cornea to cause applanation. As the cornea is deformed by the fluid pulse, an optoelectronic system monitors the cornea by detecting corneally reflected light from a beam obliquely incident upon the cornea. A peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat. During a non-contact IOP measurement, the cornea is actually deformed from its original convex state through a first state of applanation to a slightly concave state and is allowed to return from concavity through a second state of applanation to convexity as the air pulse decays.
A method for measuring IOP and a non-contact tonometer are disclosed in U.S. Pat. Nos. 6,419,631 and 6,875,175, the disclosures of which are hereby incorporated by reference in their entireties to the fullest extent allowed by applicable laws and rules. This technology is commercially known as the Reichert (Depew, New York) Ocular Response Analyzer™. According to posted information accessible at http://ocularresponse.reichertoi.com, the Reichert Ocular Response Analyzer utilizes a dynamic bidirectional applanation process to measure a cornea tissue property called corneal hysteresis. The term corneal hysteresis refers to the difference in pressure values of the air pulse at the inward moving applanation point and the outward moving applanation point during a measurement interval (inward moving refers to an initial convex corneal shape moving to a flattened condition, while the outward applanation point refers to the post air pulse concave corneal surface moving towards the applanation point on its return to a normal convex surface shape). Since corneal hysteresis appears to be a repeatable measurement, it may provide a metric that is useful for identifying and categorizing various conditions of the cornea. For example, measurement of corneal hysteresis is alleged to aid in identifying and classifying conditions such as corneal ectasia and Fuch's Dystrophy, and as helping in the diagnosis and management of glaucoma. Differences in hysteresis measurements for different corneal conditions may better inform about the biomechanical and biodynamical properties of the cornea. Because corneal hysteresis measurement is credited for presenting a complete characterization of the cornea's biomechanical state, it is believed to have additional potential uses in screening refractive surgery candidates as well as predicting and controlling surgical outcomes. The interested reader is directed to the aforementioned website address for further information provided by the manufacturer.
In view of the foregoing described techniques, capabilities and apparatus for measuring corneal parameters such as topography characteristics and hysteresis, for example, the inventor has recognized that additional benefits could be obtained by a combination of the techniques and integration of the different apparatus. The inventor has further recognized the need for new and improved methods and apparatus that are capable of more efficiently measuring properties of the cornea, resulting in a better understanding of corneal biomechanics and biodynamics.